Ossification of the Posterior Longitudinal Ligament

Gregory Gebauer, Maurice Goins, and Alan S. Hilibrand

Ossification of the posterior longitudinal ligament (OPLL) is a condition in which bone forms within the posterior longitudinal ligament (PLL), which lies posterior to the vertebral bodies within the spinal canal. The PLL is ~ 1 to 2 mm thick, consisting of a deep and a superficial layer. It traverses the posterior surface of the vertebral bodies from the cervical spine to the sacrum. As it transverses the vertebral column, it has strong attachments to the vertebral bodies and intervertebral disks. It is at these sites of “traction” that the PLL will first hypertrophy and, in many cases, partially or completely ossify. This growth of the PLL with bone and ligament may cause compression of the spinal cord. OPLL is more prevalent in the Asian population, with rates ranging from 1.9% to 4.3% in the Japanese population. It presents more often in the fifth and sixth decades of life, with a male-to-female preponderance of 3.5 to 1. Patients with OPLL are at higher risk for spinal cord injury following trauma due to the stiffness in their neck and pre-existing pressure on the spinal cord.

The exact etiology of OPLL is currently unknown, although there are multiple theories. It has been postulated that there may be a genetic factor, given that the siblings of patients with OPLL who share a greater number of human leukocyte antigen (HLA) haplotypes are at an increased risk for developing OPLL. In addition, it has been observed that excessive weight gain between the ages of 10 and 40 is also an independent risk factor for OPLL, and there is an association between OPLL and diabetes mellitus. Other studies have suggested that elevated blood levels of markers for bone formation may indicate patients pre-disposed to forming OPLL. Finally, associations between OPLL and other bone-forming diseases, such as diffuse idiopathic skeletal hyperostosis (DISH), have been suggested.

There are four different patterns of OPLL that can be identified. The first pattern is continuous, in which the ossification proceeds in a flowing, uninterrupted fashion along the posterior border of the vertebral border of the vertebral body, spanning the retrovertebral space. The second pattern is noncontinous ossification, which appears only adjacent to each vertebral body and is often referred to as segmental. The third type is localized, where only one area of ossification is identified adjacent to a single vertebra. The forth type is “mixed” and contains features of each of these aforementioned three types.

Workup

History

OPLL of the cervical spine may present with a spectrum of findings. Many patients are asymptomatic and are unaware that they have OPLL. The natural history of the disease in these individuals is unknown; however, it is known that even patients who are asymptomatic are at higher risk of spinal cord injury following a trauma. When OPLL causes symptoms, the initial presentation can be that of neck pain, arm pain, and/or weakness. Patients may also report symptoms of cervical myelopathy, including loss of dexterity or clumsiness of the hands and difficulty walking. With increasing ossification of the PLL, patients may begin to notice a decrease in range of motion of the neck. Among the majority of these patients who present with myelopathy, the severity of the symptoms can be graded using the Japanese Orthopedic Association (JOA) scoring system.

A detailed history obtained from the patient at the time of initial presentation should clarify the spectrum and duration of symptoms. Key complaints include myelopathic symptoms, such as difficulties with dexterity and manipulating small objects, worsening handwriting, problems with balance and gait, and alterations in bowel/bladder function. In addition, radicular patterns of sensory loss or motor weakness may be identified.

Physical Examination

A careful physical exam should elucidate any signs of myelopathy as well, such as disturbance in gait, loss of manual dexterity, and the presence of hyperreflexia, especially in the lower extremities. Abnormal reflexes may also be present, such as Hoffmann sign, Babinski reflex, reversed radial reflex, finger-escape sign, or sustained ankle clonus. Although these findings are classically associated with spinal cord compression, some patients with extensive cord compression due to OPLL nevertheless exhibit few of these findings. This may be due to the very slow growth of OPLL over time.

The presence of OPLL can be confirmed with radiographic evaluation. Plain radiographs are commonly used in the initial assessment. The four different patterns of OPLL (continuous, segmental, localized, and mixed) can be identified on lateral radiographs. Narrowing of the spinal canal can be seen on radiographs in areas where the ossification is significant. It can be seen anywhere from the levels of C2 to C6. OPLL can also be seen at the thoracic and lumbar levels, most commonly at the levels T4 to T7 and L1–L2. Below the level of the conus medullaris, OPLL is rarely clinically significant.

Computed tomography (CT) is the most useful radiographic modality in evaluating OPLL. It is particularly effective in defining the extent of the ossification and the resulting stenosis. Myelography is often used as an adjunct with CT scans to provide better definition of the neural elements and the compressive effects placed on these structures. However, myelography is an invasive procedure and carries with it risks of infection, dye allergy, and spinal headache if a cerebrospinal fluid (CSF) leak occurs.

Magnetic resonance imaging (MRI) is able to define the neural elements while remaining noninvasive and precluding the need for patient radiation. MRI is also able to identify subtle parenchymal changes within the spinal cord and nerve roots. However, the typical lesions in OPLL are not well defined on T1- or T2-weighted scans, thus making MRI a poor test for diagnosing OPLL.

Treatment

Nonoperative management of OPLL is appropriate for those patients with neither symptoms nor physical findings of cervical myelopathy. These patients, whose only complaint is usually neck pain, may be helped with a course of physical therapy and anti-inflammatory medications. In addition, a judicious trial of cervical traction may benefit patients who present with radicular complaints. However, traction is not appropriate for patients with progressive myeloradiculopathy. Operative treatment is generally recommended for patients with any degree of significant myelopathy or radicular symptoms unresponsive to nonoperative measures. These patients usually have a JOA score of 12 or lower (maximum score of 17). The fundamental goal may be best achieved through an anterior, posterior, or combined decompressive procedure.

Although most patients with myelopathy are safely decompressed via an anterior approach, anterior decompressive procedures in the presence of OPLL have an additional high risk of dural injury due to adherence of the OPLL to the dura anteriorly. As a result, many prefer a posterior decompression in patients with severe OPLL, especially when it occurs in a continuous fashion. Posterior approaches provide an indirect decompression of the spinal cord via cervical laminectomy, which is usually performed in conjunction with a fusion procedure, or laminoplasty. Cervical laminectomy involves complete removal of the cervical lamina across the involved segments. Laminoplasty entails enlargement of the spinal canal by hinging the lamina open on one side and securing it in an open position.

Houten and Cooper showed in their retrospective review that cervical laminectomy with fusion not only halted the progression of disease but also showed significant improvement in JOA scores in 97% of patients. Excellent recovery rates have also been reported for laminoplasty. Laminoplasty has been shown in at least one comparative study to be superior to laminectomy and fusion, although that study was not specific to patients with OPLL. However, Hirabayashi et al. have shown laminoplasty to be at least as effective as laminectomy in treating OPLL. Cervical kyphosis precludes the use of posterior surgery because the spinal cord is not able to shift away from the anterior compressive lesions.

Complications

Decompressive surgical treatment of cervical spinal cord compromise, whether performed anteriorly or posteriorly, carries inherent risk present in all surgery, such as infection, bleeding, deep vein thrombosis/pulmonary embolism, and complications from anesthesia. Specific risks with cervical surgery may be damage to nearby anatomic structures, such as the trachea, esophagus, and great vessels within the neck. Anterior, posterior, or circumferential decompressive procedures performed on or around the spinal cord in particular carry specific risks, such as paralysis, CSF leak, nerve damage leading to paresthesia or motor deficits, continuation of symptoms, axial pain, and reduced neck range of motion. Hilibrand et al. demonstrated that patients with OPLL have a significantly higher risk of neurophysiologic changes and possible spinal cord injury than other patients undergoing cervical spine decompressive procedures. Intraoperative neurophysiologic monitoring may allow early recognition of an impending neurologic injury. Early recognition of these insults may allow early institution of intraoperative maneuvers, which may curtail or reverse evolving spinal cord injuries.

A complication unique to patients with long-standing cord compression due to OPLL and severe spondylosis is the presence of thinning or complete erosion/absence of the dura. This absence of dura is usually recognized after the ossified longitudinal ligament has been removed. It is often discrete and well demarcated, with the spinal cord and nerve roots being clearly visible through the remaining arachnoid layer. This can ultimately result in CSF leakage or a CSF fistula. The limited exposure often precludes a direct repair and requires an alternative technique, such as the use of sealants (such as fibrin glue), fascial grafts, or even subarachnoid drains.